It is known in the art to combine polypropylene with an olefin copolymer such as an ethylene-propylene copolymer and an ethylene-1-butene copolymer, in order to improve impact resistance of the polypropylene. As a process for producing such an olefin copolymer, JP 3-205406A discloses a process for producing an ethylene-propylene random copolymer rubber by copolymerizing ethylene with propylene with use of a solid catalyst component obtained by contacting (i) a homogeneous solution obtained by contacting diethoxymagnesium with tetrabutoxytitanium, (ii) a specified electron-donating compound, and (iii) titanium tetrachloride, with one another.